1. Field of the Invention
The present invention relates to an RTK-GPS survey system that transmits and receives control commands and correction data by using a network.
2. Description of the Related Art
As one of the interference positioning systems that precisely measure a relative location of two observational stations by using a positioning artificial satellite such as a GPS, there is a well-known kinematic positioning system (RTK-GPS survey system). See, for example, Japanese Patent Publication 2002-311124.
In the kinematic positioning system, there are two observational points, one of which is served as an already-known reference point, and the other of which is used as an unknown observational point. The observational stations located at the two observational points simultaneously receive radio waves from an artificial satellite such as a GPS to thereby measure the relative location thereof with high accuracy. Accordingly, this allows the kinematic positioning system to determine the positional coordinates of the unknown observational point from the positional coordinates of the already-known reference point. The kinematic positioning system afterward performs analysis and processing of the signals recorded at the observational points and determines the positional coordinates.
As a further advanced model of the kinematic positioning system, there is a real-time kinematic positioning system (hereafter, referred to as an RTK positioning system).
In the RTK positioning system, one observational station is fixedly located as a base station at the reference point whose coordinates are already known of the two observational points. The other observational station is located as a rover station at the observational point whose coordinates are unknown. The base station transmits observational data to the rover station, and the rover station receives the observational data transmitted from the base station as well as the positioning satellite signals. At the same time, the RTK positioning system performs analysis and processing to thereby determine the positional coordinates of the rover station in real-time.
In concrete, in the RTK positioning system as shown in FIG. 1, a base station M1 is located at an observational point A whose coordinates are already known as a reference point. A rover station M2 is located at an observational point B whose coordinates to be sought are not yet known. After finishing measurements, the RTK positioning system moves the rover station M2 as required to another observational point whose coordinates to be sought next are not yet known. The base station M1 and the rover station M2 receive positioning satellite signals (radio waves) from an artificial satellite Sa. Referring to the base station M1, the rover station M2 performs analysis and processing in real-time simultaneously with the reception of the above signals. Thus, the RTK positioning system sequentially seeks the positional coordinates of the observational point B, which are not yet known.
With respect to the RTK positioning system, an area in which the rover station M2 can refer to a specific base station M1 (hereunder, referred to as the base station reference area) is about an radius 10 km, with the base station M1 placed at the center thereof.
This is because an excessive distance between the base station M1 and the rover station M2 will make it impossible to ignore the influences of differences in the ionosphere and atmospheric layer at the observational point, which will lead to a deterioration of measurement accuracy.
On the other hand, in order to make it possible that the rover station M2 refers to correction data of the base station M1, it is necessary to transmit the correction data to the rover station M2 from the base station M1. This transmission requires a device that transmits the data from the base station M1 by a radio transmission of a specified frequency. Because of this, the base station M1 is provided with a transmission device (a transmitter Se having the output power of about 10 mW and the frequency of 400 MHz, for example), to always transmit the correction data. A radio receiver Sc capable of receiving the radio waves from the transmitter Se is installed on the side of the rover station M2, so that the rover station M2 can refer to the correction data transmitted.
As shown in FIG. 2, there is a well known satellite positioning system using a satellite positioning data server Dsb as data transmission/reception media.
Connected to the satellite positioning system, by way of the GPS positioning data server Dsb, are at least one rover station M2, a plurality of base stations M1 and M1′, and communication devices Sx and Sy that establish communications between the rover station M2 and the base stations M1 and M1′.
In the satellite positioning system, as a common practice, the base stations M1 and M1′ are fixedly located at positions whose coordinates are already known. The base stations M1 and M1′ receive radio waves from the artificial satellite Sa continuously or periodically, and acquire the correction data of the positions in which they are located.
The measured correction data are transmitted continuously or periodically to the GPS positioning data server Dsb by the communication device Sx. For this purpose, the following is the necessary conditions: the communication device Sx as the communication interface transmits the correction data at a high speed, and includes the base stations M1 and M2′ that are located fixedly at already known positions. Accordingly, the communication device Sx as the communication interface is used in continuous connection with an exclusive network such as the WAN.
When the correction data are delivered by radio, the radio frequencies used for transmitting the correction data are generally set to be different at each of the base stations M1 and M1′. The reason is that when the radio coverage borders are adjacent, the base stations are difficult to be identified, which causes a measurement error. When the radio coverage area has an overlap, there occur radio interferences in the overlapped area. In this case, a description is made of two base stations M1 and M1′. However, the same description holds good for over three base stations.
When there is a plurality of base stations to be referred to (M1 and M1′, for example), the radio transmitters M11 thereof are set to individually different frequencies. Therefore, it becomes necessary to adjust the reception frequency of the rover station M2 in accordance with the base station used. Referring to the survey task plan, a desirable base station is generally selected among the base stations M1, M1′, . . . , and the frequency of the desirable base station is checked and adjusted before the task. Accordingly, when the rover station M2 moves over a referable arca of a base station (M1, for example) into a referable area of another base station (M1′, for example), the rover station M2 is bound to refer to the base stations M1, M1′, . . . , which are different.
Therefore, the task should be always performed recognizing the relationship between the current position of the rover station M2 and the position of the base station (M1 or M1′, etc.), which is inconvenient.
Among the correction data delivered from the base stations M1, M1′, . . . , a base station should be selected that is usable based on the number of satellites common to the base stations and rover stations and is in good condition for receiving the artificial satellite Sa. Then, the reception frequency should be appropriately set in accordance with the selected base station. This is also inconvenient.
Further, in the case of a radio transmission, the communication is confined to one direction from the base stations M1, M1′, . . . , to the rover station M2. Therefore, it is impossible to receive or transmit bi-directional data such as data for checking the condition of the rover station M2 from the base station M1. This is also inconvenient.
In the case of the Internet, there is a possibility of illegal access to the use of the network.
Further, in case of a communication through a network, fixed addresses are needed to choose a communication partner. The fixed use of the IP address is finite, and obtaining it is limited and expensive.
In case of using a general provider, the IP address is dynamic and is modified at each access; therefore, the IP address cannot be used as a fixed address in selecting to connect a partner, which is inconvenient.
The present system only needs an inherent ID in the IP-VPN, and it is possible to use an inherent number for a machine ID as an address and to designate a connection destination.
The indirect observation method based on the public survey task manual (Geographical Survey Institute of Japan, technical data A1-No. 228, June 2000, http//psgsv.gsi.go.jp/koukyou/rtk_manual/htm/mokuji.htm) requires delivering the correction data from one base station to two rover stations. Therefore, the task cannot be performed through a cellular phone network, which is inconvenient.
For the foregoing reasons, there is a need for a survey system that can overcome the inconveniences.